A customer shops at a Wal-Mart store in Beijing, February 18, 2014. REUTERS/Kim Kyung-Hoon

SHANGHAI (Reuters) - A unit of conglomerate China Resources Corp plans to sell minority stakes it holds in 21 Wal-Mart Stores Inc (WMT.N) outlets around China, a spokesman for the U.S. retailer told Reuters on Friday.

State-backed China Resources SZITIC Trust Co Ltd has listed its stakes in the stores as being for sale for a combined 3.3 billion yuan ($525 million), according to official postings from the firm on the Shanghai United Assets and Equity Exchange.

The postings didn’t make clear the reason for selling the stakes - almost all of which account for 35 percent of the jointly owned outlets - and China Resources did not respond to phone calls and emailed requests seeking comment.

The sale comes as Wal-Mart, the world’s largest retailer, and other largely brick and mortar retailers face intense competition in China from fast-developing online rivals just as growth in the world’s second-largest economy cools.

Wal-Mart, France’s Carrefour SA (CARR.PA) and Britain’s Tesco PLC (TSCO.L) have all seen sales growth slip over the last five years in China, losing market share to local rivals, according to consumer analytics firm Kantar Worldpanel.

A China-based spokesman for Wal-Mart said the firm respected the “investment decision” of its partner, adding the sale should not impact the retailer’s ongoing business in the market.

“Wal-Mart believes that the transfer of minority interest will not influence Wal-Mart’s operation and development in China,” spokesman David Fu said in emailed comments to Reuters. Fu didn’t comment on who might potentially buy the stakes up for sale.

The notices, dated last week but carried by domestic media earlier on Friday, listed the stakes for sale at prices ranging from as little as 1 yuan and up to 2.6 billion yuan. The stores were located from China’s western Sichuan province to Beijing.

Wal-Mart has been trying to boost its online presence in China and reduce reliance on physical stores. The retailer took full ownership of local e-commerce firm Yihaodian.com in July, buying the 49 percent stake it did not already own.

The U.S. retailer has been struggling to shore up its profit margins globally, which have been weighed down by a $1 billion investment announced earlier this year to increase wages for half a million store-level workers, along with other cost pressures. The company’s stock is down around 22 percent so far this year.